Inductive proximity sensor

ABSTRACT

An inductive proximity sensor and a method comprising a transmitter coil, a receiver coil, an excitation device which is connected to the transmitter coil and an evaluation device, wherein the evaluation device is designed to generate an output signal which depends on a voltage ratio between a transmission voltage of the transmitter coil and/or of the excitation device and a reception voltage of the receiver coil, wherein the excitation device is designed to generate a sinusoidal radio frequency transmission voltage. An inductive proximity sensor and a method comprising a transmitter coil, a receiver coil, an excitation device which is connected to the transmitter coil and an evaluation device, wherein the evaluation device is designed to generate an output signal which is dependent on a reception voltage of the receiver coil, wherein the excitation device is designed to generate a sinusoidal radio frequency transmission voltage with a constant amplitude.

The present invention relates to a proximity sensor, in particular to aninductive proximity sensor.

Such inductive proximity sensors serve for the recognition of a body, inparticular a metallic body or a body with good electrical conductivity,called a target, which is introduced into a region of influence of theproximity sensor. Conventional proximity sensors comprise an oscillatorwhich includes a coil which generates a magnetic field with anoscillating oscillator. In a target to be detected which is introducedinto the magnetic field, eddy currents are induced which generate acounter-field directed against the magnetic field and thus withdrawenergy from the oscillator. The oscillator in the known proximitysensors is dimensioned so that this energy withdrawal results in thecollapse of the oscillation of the oscillator or in a specific amplitudeat a specific target distance. This procedure is then detected for thepurpose of the detection of the target.

Such proximity sensors as a rule have a switching distance which is toosmall for specific applications. The switching distance designates themaximum distance of a target from the proximity sensor at which adetection of the target by the sensor is still reliably possible.

Since ferromagnetic and non-ferromagnetic materials differ greatly inthe influencing of known proximity sensors, targets of differentmaterials result in different switching states. This is unwanted anddisadvantageous in many applications.

It is the underlying object of the invention to provide an inductiveproximity sensor which has an elevated sensitivity, which allowsincreased switching intervals with respect to the prior art, whoseswitching intervals are independent of the target material and which canmoreover be manufactured at reduced costs.

The object is satisfied by an inductive proximity sensor, by atransmitter coil, by a receiver coil, by an excitation device which isconnected to the transmitter coil and by an evaluation device which isconnected to the transmitter coil and/or to the excitation device aswell as to the receiver coil, wherein the evaluation device is designedto generate an output signal which depends on a voltage ratio between atransmission voltage of the transmitter coil and/or of the excitationdevice and a reception voltage of the receiver coil, wherein theexcitation device is designed to generate a sinusoidal radio frequencytransmission voltage.

The object is further satisfied by an inductive proximity sensor, by atransmitter coil, by a receiver coil, by an excitation device which isconnected to the transmitter coil and by an evaluation device whichconnected at least to the receiver coil, wherein the evaluation deviceis designed to generate an output signal which is dependent on areception voltage of the receiver coil, wherein the excitation device isdesigned to generate a sinusoidal radio frequency transmission voltagewith a constant amplitude.

The object is further satisfied by a method for detecting metallictargets, by a transmitter coil for generating a magnetic field, by areceiver coil which is influenced by the magnetic field, by anexcitation device which excites the transmitter coil and by anevaluation device which evaluates the transmitter coil and/or theexcitation device as well as the receiver coil, wherein the evaluationdevice generates an output signal which depends on a voltage ratiobetween a transmission voltage of the transmitter coil and/or of theexcitation device and a reception voltage of the receiver coil, whereinthe excitation device generates a sinusoidal radio frequencytransmission voltage.

The object is further satisfied by a method for detecting metallictargets, by a transmitter coil for generating a magnetic field, by areceiver coil which is influenced by the magnetic field, by anexcitation device which excites the transmitter coil and by anevaluation device which evaluates at least the receiver coil, whereinthe evaluation device generates an output signal which depends on areception voltage of the receiver coil, wherein the excitation devicegenerates a sinusoidal radio frequency transmission voltage with aconstant amplitude.

Especially wide distance measurements and high sensitivities areachieved on the basis of this design of the inductive proximity sensorin accordance with the invention, wherein the distance measurement islargely independent of the conductive target material and is thuslargely material-independent. The ratio of the amplitude of the measuredreception voltage to the amplitude of the transmission voltage or themeasured reception voltage at a constant transmission voltage is ameasure for the target distance. A continuous distance measurement, andnot only the determination of a switching point, is possible by thevoltage ratio formation or by the measurement of the reception voltage.

Since only a voltage ratio from reception voltage and transmissionvoltage is formed or since only the reception voltage is measured at aconstant transmission voltage, the result is independent of a referencevalue such as with resistors or capacitors whose values can varyconsiderably due to aging or temperature influences.

Before looking at the operation of the inductive proximity sensor inaccordance with the invention, its components should first be explainedin more detail.

The inductive proximity sensor in accordance with the invention has atransmitter coil which is electrically connected to the excitationdevice and which is excited by it.

Within the framework of the invention, the excitation device cancomprise one unit or also a plurality of components electricallyconnected to one another and spatially separate. The excitation devicecan additionally have a single voltage source for exciting thetransmitter coil.

The transmitter coil is inductively coupled with the receiver coil. Thetime-variable magnetic fields which are generated by the excitation ofthe transmitter coil thereby induce a voltage in the receiver coil, thereception voltage.

Furthermore, in accordance with the invention, the evaluation device isprovided which is electrically connected to the transmitter coil and/orto the excitation device, on the one hand, as well as to the receivercoil, on the other hand, and receives and evaluates a transmissionvoltage of the transmitter coil and/or of the excitation device as wellas a reception voltage of the receiver coil such that a voltage ratiobetween the transmission voltage and the reception voltage is generatedand evaluated. This signal evaluation then generates an output signalwhich is dependent on the voltage ratio and is thus dependent on acoupling and dependent on the distance of the target.

In the independent solution in which a constant transmitter voltage isused, the evaluation device is only connected to the receiver coil.

It has been recognized in accordance with the invention that a voltageratio between the reception voltage induced in the receiver coil and thetransmission voltage occurring in the excitation device and/or in thetransmitter coil varies proportionally to the distance of a target whenthe transmission voltage is a radio frequency voltage. Such a voltageratio can be detected by the evaluation device of the proximity sensorswhich thus delivers exact distance information. The same applies to themeasured reception voltage with a constant transmission voltage.

It has been found in accordance with the invention that coupling factorsbetween the transmitter coil and the metallic target of differentmaterials approximate one another at radio frequencies. The measuredvoltage ratio then indicates a value which is proportional to thedistance and which is almost independent of the metallic material of thetarget. Especially the difference between the ferromagnetic andnon-ferromagnetic materials has no effect on the voltage ratio and thusno effect on the distance thereby determined.

The voltage ratio or the reception voltage can be evaluated digitally orby analog means. With an analog voltage ratio formation, the signal cansubsequently be digitized for further evaluation.

The sinusoidal radio frequency transmission voltage advantageously has afrequency of 0.5 MHz to 5 MHz, in particular 1 MHz to 3 MHz. In thefrequency range from 0.5 MHz to 5 MHz, the coupling factors between thetransmitter coil and the metallic target of different materialsapproximate one another and simultaneously, in this frequency range, amaximum reception voltage variation is present which results in amaximum sensitivity. In the preferred range from 1 MHz to 3 MHz, theexcitation device and the evaluation device have a particularly simpleand inexpensive structure.

In a further development of the invention, the evaluation device isformed by an analog/digital converter, wherein the reception voltage isconducted to an analog/digital converter input and the transmissionvoltage is connected as a reference voltage to the analog/digitalconverter. The evaluation device is designed particularly simply by theuse of an analog/digital converter since the transmission voltage isconnected directly as a reference voltage to a reference voltage inputof the analog/digital converter.

In accordance with a further embodiment, the evaluation device is formedby an analog/digital converter and by a division unit, wherein both thetransmission voltage and the reception voltage can be digitized usingthe analog/digital converter and the division unit is provided fordividing the digital values of the transmission voltage and thereception voltage. Since the analog/digital converter digitizes thereception voltage and the transmission voltage after one another, thatis alternately, only a single analog/digital converter is necessary. Inthis respect, conversion errors of the analog/digital converter have anequal effect on the digital values of the transmission voltage and ofthe reception voltage. The digital values of the transmission voltage orreception voltage are buffered in a memory so that both values areavailable for the division unit.

In accordance with a preferred embodiment, the evaluation device isalternatively formed by two analog/digital converters and one divisionunit, wherein the transmission voltage can be digitized by a firstanalog/digital converter, the reception voltage can be digitized by asecond analog/digital converter and the division unit is provided fordividing the digital values of the first analog/digital converter and ofthe second analog/digital converter. The formation of the voltage ratioof transmission voltage and reception voltage is simple due to thedigital division unit. The digital values are simple to process and thevoltage ratio can advantageously be output as a digital value as anoutput signal. The digital values can, for example, be output by aninterface such as an I/O link in which switching signals and messagescan be output on a common line.

In a further development of the invention, the excitation device isformed by the transmission coil with a capacitor to make an LCoscillator which generates the transmission voltage. The transmissionvoltage can be generated in a very simple manner by the use of the LCoscillator as a transmitter oscillation circuit and simultaneouslyenergy can be saved since a large part of the energy is stored in thetransmitter oscillation circuit.

In a further development of the invention, a voltage regulator ispresent which regulates the transmission voltage to a constant value.The reception voltage is thereby only dependent on the coupling factorsand on the inductances of the transmitter coil and of the receiver coiland is not influenced by any additional reference element.

In a further preferred embodiment, the evaluation unit is formed by a ΣΔmodulator. The reception voltage of the reception inductance is suppliedto the input of the one ΣΔ modulator and is conducted from this througha filter, called a loop filter in the following, to an A/D converter atwhose output a signal is applied which provides information on thedistance of the target. A very sensitive evaluation and a highresolution of the reception voltage can be carried out by the use of aΣΔ modulator. It is thus possible, for example, to detect and evaluatevoltage changes in the range from 10-6 of the reception voltage. Thehigh resolution is facilitated due to the noise shaping property of theΣΔ modulator, by which interference influences (e.g. white noise) can belargely suppressed.

There are shown in the drawing:

FIG. 1 a first inductor sensor in accordance with the invention;

FIG. 2 a second inductor sensor in accordance with the invention;

FIG. 3 an equivalent circuit diagram of a transmitter coil and of ametallic target;

FIG. 4 an equivalent circuit diagram with a transmitter coil, a receivercoil and a metallic target; and

FIG. 5 an equivalent circuit diagram in accordance with FIG. 2, with thetransmitter coil being integrated in an LC oscillator.

In the following Figures, the same parts are marked by the samereference numerals.

FIG. 1 shows a first inductive proximity sensor 1 in accordance with theinvention having a transmitter coil S, a receiver coil E, an excitationdevice 2 which is connected to the transmitter coil, and an evaluationdevice 4 which is connected to the transmitter coil S and/or to theexcitation device 2 as well as to the receiver coil E. The evaluationdevice 4 is designed to generate an output signal which depends on avoltage ratio between a transmission voltage U_(S) of the transmittercoil S and/or of the excitation device 2 and a reception voltage U_(E)of the receiver coil E. The excitation device 2 is designed to generatea sinusoidal radio frequency transmission voltage U_(S). The receptionvoltage U_(E) is in this respect influenced by a metallic target T whichshould be detected by the inductive proximity sensor 1.

FIG. 1 shows a second inductive proximity sensor 1 in accordance withthe invention having a transmitter coil S, a receiver coil E, anexcitation device 2 which is connected to the transmitter coil, and anevaluation device 4 which is connected to the receiver coil E. Theevaluation device 4 is designed to generate a output signal which isdependent on a reception voltage U_(E) of the receiver coil E. Theexcitation device 2 is designed to generate a sinusoidal radio frequencyconstant transmission voltage U_(S). As already explained with referenceto FIG. 1, the reception voltage U_(E) is influenced by a metallictarget T which should be detected by the inductive proximity sensor 1.

The equivalent circuit diagram of a transmitter coil S and of a metallictarget T of FIGS. 1 and 2 is shown in FIG. 3. The subsequent calculationshows, in accordance with the invention, how a material-independentdistance signal is generated by the inductive proximity sensor with theaid of a radio frequency in the megahertz range (MHz) and with the aidof a material-independent coupling factor.

The equivalent circuit diagram in accordance with FIG. 3 is described bythe following differential equation system, where U_(S) is thetransmission voltage, L_(S) the inductance of the transmitter coil S, M₂the counter-inductance, L_(T) the inductance of the target T, R_(T) theinternal resistance of the target, I_(S) the transmission current andI_(T) the target current.

$U_{S} = {{\frac{\mathbb{d}I_{S}}{\mathbb{d}t}L_{S}} + {\frac{\mathbb{d}I_{T}}{\mathbb{d}t}M_{2}}}$$0 = {{I_{T}R_{T}} + {\frac{\mathbb{d}I_{T}}{\mathbb{d}t}L_{T}} + {\frac{\mathbb{d}I_{S}}{\mathbb{d}t}M_{2}}}$

An equation in the frequency range with a coupling factor K₂ is obtainedby transformation of the equation system from the time range into thefrequency range by Laplace transformation and conversion of theequations according to U_(S)/I_(S).

Investigations of the inventors have shown that the followingcorrelation essential to the invention applies at high frequencies,namely that the coupling factors between the transmitter coil S and themetallic target T of different materials approximate one another at highfrequencies in the radio frequency range, that is in the MHz range.

$\omega^{2}\operatorname{>>}( \frac{R_{T}}{L_{T}} )^{2}$

The impedance of the transmitter coil S can be greatly simplified underthis condition.

$\frac{U_{S}}{I_{S}} = {{K_{2}^{2}L_{S}\frac{R_{T}}{L_{T}}} + {{j\omega}\;{L_{S}( {1 - K_{2}^{2}} )}}}$

The result shows that the imaginary portion of the coil impedance isindependent of the material parameter R_(T)/L_(T) and is only dependenton the coupling factor K₂ which is almost independent of the targetmaterial used.

A first measuring process can be derived from this. The frequency of aharmonious LC oscillator which operates at a high frequency in the radiofrequency range is a material-independent value for a target distance.This is documented by the following equation for the resonant frequencyof an LC oscillator having a capacitor C1 with a capacity C:

$f_{R} = {\frac{1}{2\pi}\sqrt{\frac{1}{{CL}_{S}( {1 - K_{2}^{2}} )}}}$

This method is still further improved as shown in the following. Theproperties of the capacitor C1 having the capacity C1, that is itsexactness, aging and temperature drift, previously had a direct effecton the measured signal. A high-resolution frequency measurementfurthermore requires a large measuring time, which restricts the signalbandwidth to be processed. For this reason, the absolute value of theimpedance of the transmitter coil S is calculated in the next step, withthe approximation of the coupling factors K₂ at high frequencies againresults in a great simplification of the impedance of the transmittercoil S.

${\frac{U_{S}}{I_{S}}} = {\omega\;{L_{S}( {1 - K_{2}^{2}} )}}$

The result shows that the absolute value of the impedance of thetransmitter coil S in the radio frequency range is likewise almostindependent of the target material.

Two improved measurement processes can be derived from this, as shown inFIG. 4. If the transmitter coil S is fed by a radio frequency sinusoidalcurrent I_(S) with a constant or target-independent amplitude, amaterial-independent measure for the target distance is obtained via themeasurement of the reception voltage U_(E). Conversely, on feeding of asinusoidal radio frequency voltage U_(S) with a constant ortarget-independent voltage amplitude, the same information is obtainedby measurement of the current.

Both measuring processes are still further improved as shown in thefollowing. The generation of a stable sinusoidal current I_(S) requiresan exact resistance whose properties, namely its exactness, aging andtemperature drift, enter into the measured result. The same appliesconversely on the voltage feed for the current measurement.

This problem can be solved with the aid of a receiver coil E inaccordance with FIG. 4 which converts the current information back intoa voltage signal again on feeding with a sinusoidal voltage U_(S). Thegreat advantage of this concept is that a material-independent measurefor the target distance, which is independent of an accurate referencevalue such as the resistance value, voltage value or frequency value, isobtained by the measurement of the reception voltage U_(E) and theforming of the ratio with the target-independent, constant or knowntransmission voltage U_(S).

The equivalent circuit diagram of this embodiment is shown in FIG. 4.The following calculation shows the function of this embodiment.

The equivalent circuit diagram in accordance with FIG. 4 is described bythe following differential equation system, where U_(S) is thetransmission voltage, L_(S) the inductance of the transmitter coil S, M₂the counter-inductance, L_(T) the inductance of the target T, R_(T) theinternal resistance of the target, I_(S) the transmission current andI_(T) the target current, U_(E) the reception voltage, M₁ acounter-inductance and M₃ a counter inductance.

$U_{S} = {{\frac{\mathbb{d}I_{S}}{\mathbb{d}t}L_{S}} + {\frac{\mathbb{d}I_{T}}{\mathbb{d}t}M_{2}}}$$0 = {{I_{T}R_{T}} + {\frac{\mathbb{d}I_{T}}{\mathbb{d}t}L_{T}} + {\frac{\mathbb{d}I_{S}}{\mathbb{d}t}M_{2}}}$$U_{E} = {{\frac{\mathbb{d}I_{S}}{\mathbb{d}t}M_{1}} + {\frac{\mathbb{d}I_{T}}{\mathbb{d}t}M_{3}}}$

By transforming the equation system into the frequency range by Laplacetransformation and conversion of the equations in accordance withU_(E)/I_(S), amount formation and subsequent simplification, after whichthe coupling factors approximate one another at high frequencies, anequation |U_(E)/U_(S)| is obtained, where K₁, K₂ and K₃ each represent acoupling factor.

$\omega^{2}\operatorname{>>}( \frac{R_{T}}{L_{T}} )^{2}$${\frac{U_{E}}{U_{S}}} = {\sqrt{\frac{L_{E}}{L_{S}}}\frac{( {K_{1} - {K_{2}K_{3}}} )}{1 - K_{2}^{2}}}$

The result shows that with this embodiment, in which the transmittercoil S is fed with a radio frequency sinusoidal voltage U_(S) which hasa known or constant voltage amplitude, an output signal is obtained atthe receiver coil E, which is picked up at high ohm, by ratio formingwith the reception voltage U_(E), said output signal representing ameasure for the target distance independent of the target material. Thissignal is only still dependent on the coupling factors K₁, K₂ and K₃ andon the inductances L_(S) and L_(E) and is not additionally influenced byany additional reference element.

It is naturally also possible to feed the transmitter coil S at thetransmitter side with a radio frequency sinusoidal current I_(S). Areference resistance would, however, again be necessary for thispurpose.

In the following, the feed of the transmitter coil S with a radiofrequency sinusoidal transmission voltage U_(S) whose amplitude is knownor constant is further improved so that this sinusoidal transmittervoltage U_(S) is relatively simple to implement and the energyconsumption is additionally very small since the feed current is stored.

For a further improvement, the transmitter coil is integrated in an LCoscillator LCO in accordance with FIG. 5. A sinusoidal voltage U_(S) isthereby generated and the energy fed in is stored in the resonantcircuit in part.

A disadvantage of a resonant circuit which is fed with a constantcurrent is that the voltage amplitude is mainly dependent on the realportion of the coil impedance. This means that the transmissionamplitude is no longer independent of the target material.

The resonant circuit is furthermore improved in that a transmitter coilS is used in which the transmitter coil S is integrated in an LCoscillator LCO whose oscillation amplitude is regulated to a constantvalue using a regulator RE and, for example, a variable resistance R.The above-calculated formula thereby applies and a material-independentoutput signal for the target distance is again obtained by a high ohmicmeasurement of the voltage amplitude of the reception voltage U_(E). Theequivalent circuit diagram of this embodiment is shown in FIG. 5.

A further possibility is to measure both the voltage amplitude of thereception voltage U_(E) and the voltage amplitude of the transmissionvoltage U_(S) and to form the ratio of these two values. This can bedone, for example, in that both values are digitized using a singleanalog/digital converter or separate analog/digital converters and aresubsequently divided. A further possibility is to use the transmissionvoltage U_(S) as a reference voltage at an ADC which digitizes thereception amplitude, which likewise corresponds to a division.

A further possibility is to use a sigma delta modulator (ΣΔ modulator)SDM as an evaluation device 2. The reception voltage U_(E) of thereception inductance LE is supplied to the input of the ΣΔ modulator andis conducted from this through a filter, called a loop filter in thefollowing, to an A/D converter at whose output a signal is applied whichprovides information on the distance of the target, which results in asuitable output signal. A high exactness becomes possibly by the noiseformation, known per se, of the ΣΔ modulator SDM by which aquantification noise is shifted to higher frequencies, whereby thewanted signal, that is the reception voltage U_(E), can be detected witha high signal-to-noise distance.

Reference Numerals

-   1 inductive proximity sensor-   2 excitation device-   4 evaluation device-   U_(S) transmission voltage-   U_(E) reception voltage-   S transmitter coil-   E receiver coil-   C1 capacitor-   L_(S) inductance of the transmitter coil S-   L_(E) inductance of the receiver coil E-   L_(T) inductance of a target-   T target-   C electric capacity of a capacitor C1-   I_(S) transmission current-   I_(T) target current-   M₁ counter inductance-   M₂ counter inductance-   M₃ counter inductance-   K₁ coupling factor-   K₂ coupling factor-   K₃ coupling factor-   R_(T) internal resistance of a target-   R variable electrical resistance-   RE regulator-   SDM Sigma Delta modulator (ΣΔ modulator)-   LCO LC oscillator

The invention claimed is:
 1. An inductive proximity sensor comprising atransmitter coil (S), a receiver coil (E), an excitation device (2)which is connected to the transmitter coil (S), and an evaluation device(4) which is connected to the transmitter coil (S) and/or to theexcitation device (2) as well as to the receiver coil (E), wherein theevaluation device (4) is designed to generate an output signal which isdependent on a voltage ratio between a transmission voltage (U_(S)) ofthe transmitter coil (S) and of the excitation device (2) and areception voltage (U_(E)) of the receiver coil (E), with the excitationdevice (2) being designed to generate a sinusoidal radio frequencytransmission voltage (U_(S)), wherein the sinusoidal radio frequencytransmission voltage (U_(S)) has a frequency of 0.5 MHz to 5 MHz,wherein the excitation device (2) forms an LC oscillator (LCO) which isdesigned to generate the transmission voltage (U_(S)) by the transmittercoil (S) with a capacitor (C), and wherein a regulator (RE) is presentwhich regulates the transmission voltage (U_(S)) to a constant value andthe regulator (RE) is a voltage regulator.
 2. An inductive proximitysensor in accordance with claim 1, wherein the evaluation device (4) isformed by an analog/digital converter, with the reception voltage(U_(E)) being conducted to the analog/digital converter input and thetransmission voltage (U_(S)) being connected as a reference voltage tothe analog/digital converter.
 3. An inductive proximity sensor inaccordance with claim 1, wherein the evaluation device (4) is formed byan analog/digital converter and by a division unit, with both thetransmission voltage (U_(S)) and the reception voltage being able to bedigitized by the analog/digital converter and with the division unitbeing provided for dividing the digital values of the transmissionvoltage (U_(S)) and of the reception voltage (U_(E)) or with theevaluation device (4) being formed by two analog/digital converters andone division unit, with the transmission voltage (U_(S)) being able tobe digitized by a first analog/digital converter, with the receptionvoltage (U_(E)) being able to be digitized by a second analog/digitalconverter and with the division unit being provided for dividing thedigital values of the first analog/digital converter and of the secondanalog/digital converter.
 4. An inductive proximity sensor in accordancewith claim 1, wherein the evaluation unit is formed by a ΣΔ modulator(SDM).
 5. An inductive proximity sensor comprising a transmitter coil(S), a receiver coil (E), an excitation device (2) which is connected tothe transmitter coil (S) and to an evaluation device (4) which is atleast connected to the receiver coil (E), wherein the evaluation device(4) is designed to generate an output signal which is dependent on areception voltage (U_(E)) of the receiver coil (E), with the excitationdevice (2) being designed to generate a sinusoidal radio frequencytransmitter voltage (U_(S)) with a constant amplitude, wherein thesinusoidal radio frequency transmission voltage (U_(S)) has a frequencyof 0.5 MHz to 5 MHz, wherein the excitation device (2) forms an LCoscillator (LCO) which is designed to generate the transmission voltage(U_(S)) by the transmitter coil (S) with a capacitor (C), and wherein aregulator (RE) is present which regulates the transmission voltage(U_(S)) to a constant value and the regulator (RE) is a voltageregulator.
 6. An inductive proximity sensor in accordance with claim 5,wherein the evaluation device (4) is formed by an analog/digitalconverter, with the reception voltage (U_(E)) being conducted to theanalog/digital converter input and the transmission voltage (U_(S))being connected as a reference voltage to the analog/digital converter.7. An inductive proximity sensor in accordance with claim 5, wherein theevaluation device (4) is formed by an analog/digital converter and by adivision unit, with both the transmission voltage (U_(S)) and thereception voltage being able to be digitized by the analog/digitalconverter and with the division unit being provided for dividing thedigital values of the transmission voltage (U_(S)) and of the receptionvoltage (U_(E)) or with the evaluation device (4) being formed by twoanalog/digital converters and one division unit, with the transmissionvoltage (U_(S)) being able to be digitized by a first analog/digitalconverter, with the reception voltage (U_(E)) being able to be digitizedby a second analog/digital converter and with the division unit beingprovided for dividing the digital values of the first analog/digitalconverter and of the second analog/digital converter.
 8. An inductiveproximity sensor in accordance with claim 5, wherein the evaluation unitis formed by a ΣΔ modulator (SDM).
 9. A method of detecting metallictargets comprising a transmitter coil (S) for generating a magneticfield, a reception coil (E), which is influenced by the magnetic field,an excitation device (2) which excites the transmitter coil (S), and anevaluation device (4) which evaluates the transmitter coil (S) and/orthe excitation device (2) as well as the receiver coil (E), wherein theevaluation device (4) generates an output signal which is dependent on avoltage ratio between a transmission voltage (U_(S)) of the transmittercoil (S) and of the excitation device (2) and of a reception voltage(U_(E)) of the receiver coil (E), with the excitation device (2)generating a sinusoidal radio frequency transmission voltage (U_(S)),wherein the sinusoidal radio frequency transmission voltage (U_(S)) hasa frequency of 0.5 MHz to 5 MHz, wherein the excitation device (2)generates the transmission voltage (U_(S)) by oscillation, and whereinthe transmission voltage (U_(S)) is regulated to a constant value.
 10. Amethod of detecting metallic targets in accordance with claim 9, whereinthe evaluation device (4) carries out an analog/digital conversion, withthe reception voltage (U_(E)) serving as an input value and thetransmission voltage (U_(S)) serving as a reference value for theanalog/digital conversion.
 11. A method of detecting metallic targets inaccordance with claim 9, wherein the evaluation device (4) carries outan analog/digital conversion and a division, with the transmissionvoltage (U_(S)) and the reception voltage being digitized in arespective time-separate analog/digital conversion, and with the digitalvalues of the transmission voltage (U_(S)) and of the reception voltage(U_(E)) being divided or with the evaluation device (4) carrying out twoanalog/digital conversions in parallel and subsequently carrying out adivision, with the transmission voltage (U_(S)) being digitized in afirst analog/digital conversion, with the reception voltage (U_(E))being digitized in a second analog/digital conversion, and the digitalvalues of the transmission voltage (U_(S)) and of the reception voltage(U_(E)) being divided.
 12. A method of detecting metallic targetscomprising a transmitter coil (S) for generating a magnetic field, areception coil (E), which is influenced by the magnetic field, anexcitation device (2) which excites the transmitter coil (S), and anevaluation device (4) which at least evaluates the receiver coil (E,)wherein the evaluation device (4) generates an output signal which isdependent on a reception voltage (U_(E)) of the receiver coil (E), withthe excitation device (2) generating a sinusoidal radio frequencyconstant transmission voltage (U_(S)) with a constant amplitude, whereinthe sinusoidal radio frequency transmission voltage (U_(S)) has afrequency of 0.5 MHz to 5 MHz, wherein the excitation device (2)generates the transmission voltage (U_(S)) by oscillation, and whereinthe transmission voltage (U_(S)) is regulated to a constant value.
 13. Amethod of detecting metallic targets in accordance with claim 12,wherein the evaluation device (4) carries out an analog/digitalconversion, with the reception voltage (U_(E)) serving as an input valueand the transmission voltage (U_(S)) serving as a reference value forthe analog/digital conversion.
 14. A method of detecting metallictargets in accordance with claim 12, wherein the evaluation device (4)carries out an analog/digital conversion and a division, with thetransmission voltage (U_(S)S) and the reception voltage being digitizedin a respective time-separate analog/digital conversion, and with thedigital values of the transmission voltage (U_(S)) and of the receptionvoltage (U_(E)) being divided or with the evaluation device (4) carryingout two analog/digital conversions in parallel and subsequently carryingout a division, with the transmission voltage (U_(S)) being digitized ina first analog/digital conversion, with the reception voltage (U_(E))being digitized in a second analog/digital conversion, and the digitalvalues of the transmission voltage (U_(S)) and of the reception voltage(U_(E)) being divided.